Semiconductor devices are built up using a number of material layers. Each layer is patterned to add or remove selected portions to form circuit features that will eventually make up an integrated circuit. To pattern a layer by removal of material, a mask is first formed on the layer using photolithographic techniques. The exposed portions of the layer that lie between the features of the patterned mask are then removed by etching. Dry etching is superior to wet etching for patterning very small circuit features. Dry etching includes chemical techniques such as plasma etching and physical techniques such as sputter etching.
A typical plasma etching apparatus includes a processing chamber which encloses the semiconductor wafers that are to be etched. In the etching process, an energy source activates fluorine-based or chlorine-based gases to form a plasma (ionized gas) containing a number of reactive species. The plasma reacts chemically with the surface layer of the wafer in areas not covered by the mask. Atoms in the surface layer combine with reactive species in the plasma to form volatile products. The volatile products depart the surface layer and are pumped away, resulting in removal of the surface layer in areas not covered by the mask.
Not all of the volatile products are pumped away, unfortunately. Some of the volatile products condense to form a film of process residue on surfaces of the processing chamber and on other parts within the plasma etching apparatus. With continued use of the plasma etching apparatus, the film of process residue can begin to flake off, releasing particles that threaten to induce defects on the wafers being processed. To eliminate these particles, the processing chamber and other parts must be periodically cleaned or replaced.
The methods and chemicals used to clean the processing chamber and other parts depend upon the composition of the process residue to be removed. To remove polysilicon, for example, a 1:1 solution of hydrofluoric acid and nitric acid is typically used. The nitric acid first reacts with the polysilicon to oxidize it and produce silicon oxide. The silicon oxide is then removed by the hydrofluoric acid. Hydrofluoric acid is also used to remove metals.
Refurbishing processing chambers and other parts for use in plasma etching entails more than removing process residue from those parts. It may also be advantageous to repair any damage to the surface of the parts caused by exposure to highly reactive plasmas. The processing chamber typically is made of aluminum, glass or quartz. Quartz is also commonly used for other parts of the plasma etching apparatus. A plasma that is sufficiently reactive to etch silicon wafers will also etch quartz, which is a form of silicon dioxide. Plasma etching erodes quartz, changing a smooth transparent surface to a rough surface having a “frosted” appearance.
Etched quartz tends to release small particles from its surface, a process that is called spalling or spallation. Quartz is formed of small crystallites joined in a matrix. When quartz is etched, the etching proceeds faster in localized areas between the crystallites, resulting in undercutting of the unetched crystallites. Those crystallites are then liberated from the matrix and released as particles. As in the case of process residue particles, quartz particles can cause defects in the wafers to be processed.
Firepolishing is generally used to restore etched quartz surfaces. When firepolishing a part made of quartz or any glass, the aim is to melt the surface of the part without melting the bulk of the part underlying the surface. A flame is often used in firepolishing, but is not required. Localized surface melting reduces the microscopic roughness of the surface, thereby reducing the release of quartz particles, and restores a smooth transparent appearance.
Refurbishing of plasma etching apparatus parts typically includes removal of process residue, firepolishing, and a final cleaning with hydrofluoric acid. As noted above, hydrofluoric acid, alone or in combination with other agents, is commonly used to clean quartz parts. For cleaning quartz, hydrofluoric acid is typically used at a concentration of 5 to 15 percent. Hydrofluoric acid is favored for cleaning quartz because it removes a wide variety of contaminants, including certain metal contaminants such as aluminum which are difficult to remove. One reason why hydrofluoric acid is effective at cleaning quartz is that hydrofluoric acid reacts chemically with the surface of the quartz, resulting in removal of silicon and oxygen atoms from the surface. Removal of these atoms from the surface of the quartz also dislodges contaminants adhered to those surface atoms.
Hydrofluoric acid is commonly used in the manufacturing of new quartz parts, both for cleaning subparts at intermediate stages of assembly and for removing manufacturing contaminants from newly manufactured quartz parts. Surfaces of new quartz parts commonly bear aluminum and other contaminants which can damage silicon wafers, so the new quartz parts typically are cleaned thoroughly before they are used in semiconductor processing.
Quartz parts are used in many types of semiconductor processing besides plasma etching processes. For example, quartz parts may be used in chemical vapor deposition (CVD) and physical vapor deposition (PVD) processes. Process residue, including metal contaminants, may accumulate on quartz parts used in various types of semiconductor processing. Cleaning and refurbishing may be required for quartz parts used in various types of semiconductor processing.
Both new and refurbished quartz parts typically are tested thoroughly before they are used in semiconductor processing. Testing of new and refurbished parts includes measurement of metals to ensure that metal contaminants do not exceed permitted levels. Testing of quartz parts also includes measurement of particle release in process. Unfortunately, refurbished quartz parts often show unacceptably high release of particles in process, rendering the parts unusable. Furthermore, newly manufactured quartz parts sometimes show unacceptably high release of particles. There is a need for refurbished and new quartz parts with low levels of metal contaminants and with consistently low release of particles in process.